The present invention relates to comminuting apparatuses, more particularly to comminuting elements for impact pulverizing machines, and most particularly to an element which both comminutes and releases reduced fine particles in an impact pulverizing machine.
Size reduction apparatuses are available in the marketplace for reducing the size of materials such as food products, chemicals, rubbers, and resins. For example, granulators typically cut the material from several inches to as fine as 20 mesh (U.S. Standard). It is often desirable, however, to reduce the size of the materials down to approximately 40 to 60 mesh. Impact pulverizing machines are often used to achieve this further reduction in material size. See, for example, The Bantam.TM. Mikro-Pulverizer Bulletin 51F2, MikroPul, Division of the Slick Corp., Summit, N.J. (1970). These impact pulverizers break apart materials by rendering an instantaneous blow from a moving body hitting another body.
Conventional impact pulverizing machines typically include a grinding chamber with high speed rotating beaters to break up the materials. The broken materials having a selected reduced fineness fall through a perforated thin metal screening element and pass out from the grinding chamber. Conventional screening elements for pulverizing machines are generally well-known items and are currently in widespread use. See, for example, herringbone slots, round perforations, and cross slot perforation screens in the Bantam.TM. Mikro-Pulverizer Bulletin 51F2, (supra). Conventional sheet screens, however, readily deform and tear when struck with large unreduced materials at high rates of speeds. A deformed or torn screen produces contaminated product due to larger sized particles passing therethrough. Therefore, the torn screens must be immediately replaced. In fact, it is common for users of pulverizing machines to keep a multitude of screens in stock in anticipation of constant replacement of the screens.
Jump-gap screens are also known in the marketplace, see, The Bantam.TM. Mikro-Pulverizer, Bulletin 51F2, (supra). A significant disadvantage accompanying the jump-gap screen is that it is ineffective for a variety of grinding applications. Extremely course materials often pass through the jump-gap screen before size reduction occurs. Although conventional screening elements are currently in widespread use, it would be desirable to develop an element which is durable enough to withstand impact by large materials, retains even extremely coarse materials within the grinding chamber for size reduction, and is itself formed to reduce the particle size of the materials.
The present invention is directed accordingly to a comminuting apparatus which replaces conventional screening elements used in impact pulverizing apparatuses with a grinding chamber, rotating beaters, and a support frame. A comminuting apparatus having features of the present invention comprises a series of bars circumferentially angled about the periphery of the grinding chamber generally parallel to the tangent of the rotation path of the beaters. So, a material approaching the apparatus contacts the series of bars such as a stone strikes a wall and the angle of this wall is such as to shatter the material and to deflect the material toward the rotating beaters.
The present invention which comprises the series of bars is formed so that adjacent bars are coupled together in generally parallel overlapping relationship to form a material discharge gap between the adjacent bars. Thus, a flow path for the reduced fine material is positioned generally parallel to the tangent of the rotation path of the beaters. Material which has been shattered by the beaters and/or the bars is carried by high velocity currents created by the rotating beaters through the flow path. The comminuting apparatus in accordance with the present invention may also be referred to in this description and otherwise as a combination screen for an impact pulverizing apparatus.
Various materials have different tendencies to shatter within the grinding chamber. Some materials, in fact, begin to melt and become semi-plastic with repeated energy input by repeated blows by beaters. So, it may be preferable to include a material discharge portion in the apparatus adjacent to the series of bars to discharge such undesirable materials. The material discharge portion includes curved branches extending away from the series and toward the frame and a material discharge aperture extending between the branches. Soft and gummy materials are preferably discharged from the grinding chamber because they often bounce off of the bars and the rotating beaters rather than shatter into reduced materials. This bouncing adds heat to the system and could lead to clogging of the flow path and the gumming up of the remaining material in the system. Thus, soft materials are automatically released from the grinding chamber through the material discharge aperture after approximately one pass across the series of bars.
The present invention is further directed to a method for reducing the particle size of materials in a pulverizing machine which includes an inlet end and a material discharge end. The method includes inserting the material to be reduced into the inlet end of the pulverizer. The material enters the chamber and falls into the rotation path of the beaters where it is struck with the beaters and transformed to a reduced fineness. That material is then impacted against a series of bars, each bar having an impact edge generally perpendicular to the rotation path of the beaters, a radially inner side circumferentially angled about the periphery of the grinding chamber generally parallel to the tangent of the beating means, and an opposite radially outer side.
After impacting the bars, the material having the selected reduced fineness passes between the adjacent bars and exits the pulverizer through the discharge end. The materials having a size greater than that of the selected reduced fineness, however, are deflected off of the impact edge and into the rotation path of the beaters to repeat the grinding procedure. Since it is necessary that the materials be embrittled to shatter them in the grinding chamber, it may be preferable to cool organic materials until they are near or below the glass transition temperature (Tg) of said organic material. The material is contacted with a cooling compound, preferably gaseous or liquid nitrogen, or gaseous or solid CO.sub.2 "dry ice" before being placed into the grinding chamber.
Other objects and features of the present invention will become apparent as this description progresses.